A variety of plastic films are used in the packaging industry for storage and shipping of goods. In many cases these plastic films are printed with text and images. Printing inks used for this purpose must meet certain end-use requirements for successful use in this application. These requirements include good printability, resolubility, and resistance properties. Inks used with plastic packaging must also exhibit good wetting and flow properties and fast drying times.
Due to environmental and health concerns from solvent based inks, increased emphasis has been placed on the use of water-based inks for printing on films used for packaging in recent years. Water-based inks with good printability, i.e., good ink transfer and wetting, good adhesion to substrate and good image resolution are therefore highly desirable. Similarly, water-based inks which have good resolubility, the ability of dry ink to re-disperse in the same ink when in a wet state, are also important. For example, if a press is stopped, as water-based inks begin to dry on the rollers they can undergo physical and chemical changes due to film formation, change in pH, and the like which render the printability of these inks problematic. Finally, resistance of the printed or coated film to mechanical forces, water, solvents and other chemicals is also highly desirable. However, many previous water-borne polymer coatings suffer from problems with adhesion, abrasion resistance, and water and solvent resistance.
Various polyvalent metal cross-linking agents have been used as additives to water-based coatings in an effort to improve the resistance properties of the coatings. In particular, zirconium-based cross-linking agents have been promoted as useful additives to water-borne polymer compositions, offering high solvent and abrasion resistance. The basis of the improved wet resistance properties is believed to be linkages formed by the metal species between the polymer molecules at carboxylate and hydroxyl groups. More specifically, it is believed that the carboxylate groups are directly bound to, e.g., the zirconium cations upon evaporation of water and other volatile constituents. The reaction is irreversible under normal coating conditions. In contrast, hydroxyl groups on the polymer likely hydrogen bond with hydroxyl groups attached to the zirconium ion. In either case, the exact nature of the metal crosslinking species (usually in the form of polymeric complexes) will depend on the amount of metal cross-linking agent present, the nature of the polymer, and the pH of the system, among other variables.
Despite their advantages, metal cross-linking agents such as zirconium agents suffer from chemical and colloidal stability problems. For example, ammonium zirconium carbonate (AZC) is susceptible to oxolation, a process in which the carbonate group is lost and an insoluble polymer gel of zirconium oxide is formed. This problem is exacerbated at dilute concentrations of AZC such as typically would be used with a water-borne polymer. In fact, a manufacturer of an aqueous solution of AZC (BACOTE 20, having 20% ZrO2 solids) recommends that AZC be added as the last ingredient to a formulation and not be mixed with any other material or otherwise diluted prior to use. Furthermore, compositions containing water-borne polymers and metal crosslinking agents such as zirconium lose their properties of resolubility and printability upon storage. Hence, the instability of zirconium agents such as AZC severely limits their use in water-based coatings.
There is a need for water-based coatings, especially inks, which have good wetting, drying speed, flow behavior, printability, resistance properties and resolubility when used for printing on plastic films and other substrates. Furthermore, such coatings must be stable upon storage and under normal conditions of use. Water-based inks and coatings of this invention exhibit these and other desirable properties when used on plastic films and other substrates.